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Octane rating

Old 01-28-2007, 09:55 AM
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Octane rating

Hi everbody! I'm new to the whole forum thing, but this page has some useful information about a great bike so I thought might want to be a part of it. Anyway I have a question about octane levels. The service manual says to use 86 octane or higher and I use premium gas and I swear it runs a little better, could be all in my head, but I think it runs a little better. Now back in my V8 tuning days before gas got so expensive I was told to run the lowest octane suggested by the manufacturer because the higher the octane the slower the fuel burns and this creates soot build up on the valves if the motor was not designed to run higher octane. Now this is something I heard not seen. My question is does this statement hold water with the superhawk, or should I use the gasoline that makes me feel better? Kind of an obscure question but I think to much!
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Old 01-28-2007, 09:58 AM
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the general consensus around here is 87 runs best for a stock motor. The higher the octane the less energy is available. It's GREAT for high compression motors cause it won't combust from the compression alone and damage your motor. Out motors are low compression and the premium gas doesn't offer any advantage to us. Some people I believe still run it for the tracks or what not. Kind've personal opinion basically
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Old 01-28-2007, 10:59 AM
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High octane in VTR = added $$$$ and less hp, hardly a desirable investment....

granted it is not a VTR, but on dyno tests conducted on a 700 cc two-stroke snowmobile engine (Ski-Doo MXZ700, designed to run on 87 octane), switching to hi-test resulted in a 2 hp drop in output! Food for thought.
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Old 01-28-2007, 01:12 PM
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Mine seems to run a bit better on 87 or 89, I still will run 93 about every 3rd or 4th tank just to switch things up a bit...
I agree this is not a high comp engine, so does not make sense to run the higher octane.....
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Old 01-28-2007, 03:36 PM
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I run a +4 degree ignition advancer so I always run 93. In those unusual times where I did run 87 I didn't notice a difference. I just have a "thing" about running good gas. Whatever makes you sleep well at night, I say. I've not heard of carbon buildup on these motors, though it does occur in the Chevy Vortec motors.
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Old 01-28-2007, 04:33 PM
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I run 89 in my hawk here...only because it was beat on by the previous owner and is out of tune, so I want to avoid any possible detonation but I know that 93 would be overkill. Once I take it to get tuned I'm switching to 87. Im sure I could run it w/o issue right now....but I sleep better knowing it's on the safe side for the time being.

And you'd be amazed how many people in the world don't have a CLUE about octane or what it REALLY means and will argue vehemently with you about how their stock motor "feels stronger" on 93 octane. I deal with this with car guys all the time. I run 93 in my car, because it has 5 degrees BTDC over stock, heads, big cam, intake, full exhaust, etc... And I dont run damn race fuel....and if I choose to back the timing off, I can run 87 all day long (only 9:1 cr)....so explain to me why your Lincoln with exhaust needs 93 octane? lol
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Old 01-28-2007, 06:25 PM
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87.
It's all I ever run and I hammer mine pretty hard.
Never any indication of preignition.

Yes, Octane is like voodoo to many folks. They don't understand it but believe it has magic power.

OK, poor analogy but you get the picture.

I sometimes hear folks say the run a tank of 93 every so often for the detergents and to clean things out.

Unless I am badly mistaken, Fedral Law requires a minimum (and I'd assume acceptable) level of detergents in all grades of gas.

But in the end, gas is a lot like oil. Folks tend to use what makes them feel better. And fortunately, most choices in either area are going to work fine.
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Old 01-28-2007, 06:35 PM
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Originally Posted by Brian A View Post
I sometimes hear folks say the run a tank of 93 every so often for the detergents and to clean things out.

Unless I am badly mistaken, Fedral Law requires a minimum (and I'd assume acceptable) level of detergents in all grades of gas.

But in the end, gas is a lot like oil. Folks tend to use what makes them feel better. And fortunately, most choices in either area are going to work fine.
Heard this before too.... If you want extra detergents....run a fuel system additive.
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Old 01-28-2007, 07:15 PM
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some cars have enough range of timing adj. in their computers to take advantage of higher octane fuels by 'running on' their knock sensors, but there are very few that have enough adj. to do this. to my knowledge there are no bikes being made with knock sensors yet and only a few are running a closed loop fuel inj. system. on a non-monitored system like our 'Hawks higher octane will only be of benefit if you're running a timing advance like Hawkrider.
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Old 01-28-2007, 08:47 PM
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Ditto.

Engines with full computer controlled ignition timing can utilise higher octanes, as the computer will compensate for the different fuel. Engines with fixed advance won't though.
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Old 01-30-2007, 10:27 AM
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I took a course called "Petroleum Refining for Non-Technical Personnel", so....

Keep in mind the octane rating is based on conditions simulated in a laboratory. Octane is the rating of the gasoline's ability to resist uncontrolled burning or "knocking".

Research Octane Number (RON)
Determined in a laboratory engine designed to simulate mild driving conditions.

Motor Octane Number (MON)
Same laboratory engine at more severe driving conditions

(R+M)/2 Antiknock Index
Numerical average of RON and MON approximates average automobile performance. This is the sticker you see on gas pumps.

Sensitivity: Difference between MON and RON
Typically this is 8 for base unleaded and 11 for premium grade gasoline.


As for the detergents used in each grade, someone mentioned the minimum required. The person who taught the course I was in said there was no difference between the grades. Basically a marketing ploy. A state of mind for the end user if you will. If you sleep better at night by using the more expensive grade then so be it.
Also keep in mind the majority of gasoline is moved through our great country via pipeline. A pipeline used by lots o' companies. Just because it says Exxon, Conoco, or whatever, their gas may be refined by an independent like Valero, etc.
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Old 01-30-2007, 12:59 PM
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you really just have to experiment with different brands and grades. and don't buy gas if the station's tanks have just been filled, stirs up the sediments in the bottom of the tank. I have found that Amoco, now BP/Amoco, gives good performance but poor mileage; Chevron seems to be good in both regards and el cheapo budget places are bad in both regards.
I have noticed that my 'Hawk will have some light detonation if I run 87 octane but I'm at sea level and so have some pretty dense air.
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